Thin film transistors (TFTs) are widely used in electronic devices, such as liquid display panels (LCDs), to serve as a switch component. Generally, a TFT can include a gate, a source, a drain, and a channel layer coupling the source to the drain. Metal oxide are widely used to form the channel layer because it's good characteristics (such as good conductivity and high electron mobility). Usually, an etching stopping layer (ESL) is utilized in the TFT to protect the channel layer.